Enteroaggregative Escherichia coli (EAEC) is an emerging enteric pathogen associated with sporadic, endemic, and epidemic diarrheal illnesses in individuals of all ages in both developing and industrialized countries. (Nataro et al., Emerg Infect Dis 4:251–261 (1998); Nataro et al., Clin Microbiol Rev. 11:142–201 (1998); Okeke et al., Lancet Infect Dis 1:304–313 (2001)). The pathogenesis of EAEC infection includes strong adherence to the intestinal mucosa, most likely to both the small and the large intestines, followed by secretion of one or more enterotoxins that induce cytopathic effects in intestinal epithelial cells. (Nataro et al., Infect Immun 64:4761–4768 (1996); Eslava et al., Escherichia coli. Infect Immun 66:3155–3163 (1998); Czeczulin et al., Infect Immun 67:2692–2699 (1999)). Adherence of EAEC to the intestinal mucosa is characterized by the presence of a thick aggregating biofilm, which may favor the persistence of this organism in the human intestine. (Nataro et al., Clin Microbiol Rev. 11:142–201 (1998); Tzipori et al., Infect Immun 60:5302–5306 (1992)). In addition, EAEC may induce intestinal inflammation, which can precipitate growth failure even in the absence of diarrhea. (Steiner et al., J Infect Dis 177:88–96 (1998)).
The defining feature of EAEC is its distinctive characteristic aggregative adherence (AA) pattern to HEp-2 cells in culture. (Nataro et al., Pediatr Infect Dis J 6:829–831 (1987)). In particular, EAEC adhere to the surface of HEp-2 cells, to the glass substratum, and to each other in a distinctive stacked-brick formation. Aggregating adherence to HEp-2 cells in well-characterized strains requires the expression of one or more members of the plasmid-bome Aggregative Adherence Fimbriae (AAF) family. (Nataro et al., Infect Immun 60:2297–2304 (1992); Czeczulin et al., Infect Immun 65:4135–4145 (1997)). Two of members of the AAF family, AAF/I and AAF/II, have been characterized at the genetic level and each is encoded on a large plasmid (designated pAA). (Savarino et al., J Bacteriol 176:4949–57 (1994); Elias et al., J Bacteriol 181:1779–85 (1999)). It has been shown that the human pathogenic strain 042 requires AAF/II fimbrial antigen for adherence of the bacterium to the colonic mucosa, thereby suggesting that this adhesin is a virulence factor for human infection. (Czeczulin et al., Infect Immun 65:4135–4145 (1997)).
It has been shown that the majority of EAEC strains lack AAF/I and AAF/II. (Czeczulin et al., Infect Immun 67:2692–2699 (1999)). Nevertheless, most EAEC strains carry the ca. 100 kb pAA plasmid, recognized by the presence of several conserved loci. The most prominent among these loci is a transcriptional activator of the AraC class designated AggR, which is required for expression of both AAF/I and AAF/II. AggR is also present in a large percentage of EAEC strains that do not express any identified AAF. (Nataro et al., J Bacteriol 176:4691–4699 (1994)). Recently, a novel AggR-dependent gene lying immediately upstream of AggR in EAEC 042 has been identified and characterized. (Sheikh et al., J Clin Invest (in press) (2002)). This AggR-dependent gene encodes a secreted 10.2 kD protein, designated Aap, that appears to coat the bacterial surface and promote dispersion of EAEC on the intestinal mucosa. Aap has alternatively been designated dispersin. It has been shown that Aap mutants form larger aggregates, fewer individual bacteria, and aggregate more intensely than the wild type. In addition, Aap partially counteracts AAF-mediated aggregation and may play a fundamental role in EAEC pathogenesis. Moreover, the data suggest that Aap binds non-covalently to the bacterial cell surface. However, the mechanism by which Aap is translocated across the outer membrane is as yet unknown.
Another prominent locus is the binding site of a DNA probe, CVD432, on the plasmid. The CVD432 probe was developed to simplify the identification of EAEC and has been used as the AA probe. (Baudry et al., J Infect Dis 161:1249–1251 (1990)). In its original evaluation, the probe was found to be 89% sensitive and 99% specific for EAEC. The nucleotide sequence of the AA probe represented a cryptic open reading frame (ORF) located adjacent to the plasmid replicon. (Nataro et al., Infect Immun 64:4761–4768 (1996)). Further sequence analysis of the AA probe region revealed five ORFs. The predicted protein of one of the ORFs was similar with the ATP-binding cassette (ABC) domain of the ABC transporter, suggesting that the gene cluster is involved in the transport of an unidentified molecule of EAEC. Furthermore, this gene cluster appears to be associated with the translocation of Aap, and the formation of a new transporting system of the virulence factor of EAEC. The five gene cluster was designated aat.
EAEC infections are medically important in populations around the world. For example, in developing countries, children are often infected with EAEC and can develop prolonged diarrhea, often with significant malnutrition. In industrialized countries, EAEC is an emerging cause of traveler's diarrhea. An effective vaccine to protect against enterotoxigenic E. coli would protect against at least one-half of traveler's diarrhea cases. However, in the past, there has been no effective treatment or prevention of EAEC-induced illnesses.
It is therefore desirable to identify and characterize Aap and the five gene cluster (aat) of the AA probe region of the pAA plasmid of EAEC 042 and to use these proteins and their corresponding nucleotide sequences for diagnosis, therapy, and prevention of EAEC infections.